1. Field of the Invention
This invention relates to assemblies having two or more bodies mated with an adhesive material reinforced by lateral compression loading, and in particular to such assemblies in which the adhesive bond strength is augmented by reducing stresses lateral to the adhesive bond due to longitudinal and/or torsional loading. This invention is especially suitable for use in strut tube assemblies and rocket assemblies.
2. Description of Related Art
Vessels and other containers capable of handling extreme axial loads and/or high internal operation pressures can be found in various technical fields and have been employed in connection with wide and diverse applications. Such vessels have been used, for example, as tubular struts and rocket motor casings.
Many of these applications require one or both ends of the vessels to be equipped with appropriate end fittings. One example of a pressurized vessel equipped with end fittings is a case assembly of a small tactical rocket assembly, which is illustrated in exploded view in FIG. 9 and generally designated by reference numeral 900. The illustrated embodiment of the case assembly 900 includes a case sleeve 902, in which a forward receptacle end 904 accommodates a mating portion 910 of a forward end fitting 908 and an aft receptacle end 906 accommodates a mating portion 914 of an aft end fitting 912. Although not shown in FIG. 9, the aft end fitting 912 can constitute, be formed integrally with, or otherwise be firmly secured to a nozzle assembly, whereas the forward end fitting 908 can likewise be connected to an aerodynamic conical member, warhead, other structural components and combinations thereof. Mechanisms useful for securing the end fittings 908 and 912 to nozzle assemblies, conical members and the like include, among others, welds, bolts, adhesive joints, screw threads, a lock wire, the like, or combinations thereof.
The end fittings 908 and 912 are connected to the case sleeve 902 via adhesive bonds. The adhesive bonds are formed by applying a polymeric or functionally comparable adhesive to and in-situ curing the adhesive at opposing bonding surfaces defined between the receptacle ends 904 and 906 of the rocket motor casing 902 and the associated mating portions 910 and 914 of the forward and aft end fittings 908 and 912, respectively.
During normal operation, shear stresses develop within the adhesive as the result of axial load differentiations (applied in the direction designated by arrow L.sub.a in FIG. 9) between the case sleeve 902 and end fittings 908 and 912 caused by, among other things, internal pressure, acceleration of the rocket assembly 900 and sudden deceleration. In addition, lap shear joints have an inherent moment because the load path is not linear. The moment causes the development of lateral (or normal) stresses in the adhesive bond system. For some applications, the adhesive bond does not possess sufficient strength to withstand the shear and normal stresses encountered in these operating conditions. In order to compensate for inherent weaknesses in the adhesive bond connecting the end fittings 908 and 912 to the forward and aft receptacle ends 904 and 906 of the case sleeve 902, it is often necessary to provide supplemental mechanical fasteners (not shown) to reinforce the adhesive bonds. Conventional supplemental fasteners include blind fasteners such as pop rivets, standard nuts and bolts, or bolts that extend through the tubular piece and thread into the end fitting. Without the provision of the supplemental mechanical fasteners, extreme shear and normal stresses acting on the adhesive bond may lead to failure at the opposing bonded surfaces with catastrophic results.
Despite the beneficial contribution of conventional supplemental mechanical fasteners as reinforcements, the use of such fasteners is often discouraged due to their expense and weight penalty. The reality of marketplace demands on minimizing costs has, to a large extent, made the use of supplemental mechanical fasteners cost prohibitive for some applications. Also, for design purposes, the exclusion of mechanical fasteners is desirable, since their presence can lower the linear design allowable because of bearing stresses in the composite case.
A need therefore exists for an assembly including a vessel connected to at least one end fitting via an adhesive bond, which adhesive bond is designed to tolerate large shear stress encountered during operation without the need for supplemental mechanical fasteners.